1. Field of the Invention
The present invention relates to an intermediate fluid type vaporizer for heating and vaporizing a low temperature liquid, such as liquefied natural gas (hereinafter referred to as xe2x80x9cLNGxe2x80x9d), by using an intermediate fluid such as propane.
2. Description of the Related Art
Hitherto, an intermediate fluid type vaporizer using an intermediate fluid in addition to a heat source fluid is known as means for continuously vaporizing a low temperature liquid, such as LNG, with a compact structure (see, e.g., Japanese Unexamined Patent Application Publication No. 53-5207).
FIG. 6 shows one example of such an intermediate fluid type vaporizer for LNG. This conventional vaporizer comprises an intermediate fluid evaporator E1, an LNG evaporator E2, and a natural gas (hereinafter referred to as xe2x80x9cNGxe2x80x9d) heater E3.
The intermediate fluid evaporator E1 comprises a first shell 101, an outlet chamber 102 formed at one end of the first shell 101, an intermediate chamber 103 formed at the other end of the first shell 101, and a large number of heat source tubes 104 disposed in a lower portion of an inner space of the first shell 101 and extending between both the chambers 102, 103. The first shell 101 contains therein an intermediate fluid (e.g., propane) having a boiling point lower than that of sea water as a heat source fluid. The LNG evaporator E2 comprises an inlet chamber 111 and an outlet chamber 112 divided from each other by a partition wall 110, and a large number of heat transfer tubes 113 for communicating both the chambers 111 and 112 with each other. Each of the heat transfer tubes 113 has a substantially U-shape and projects into an upper portion of the inner space of the first shell 101. The NG heater E3 comprises a second shell 120 provided in continuation with the intermediate chamber 103, an inlet chamber 121, and a large number of heat source tubes 122 extending between both the chambers 103, 121.
A heat source fluid (sea water in the illustrated related art) flows through the inlet chamber 121, the large number of heat source tubes 122, the intermediate chamber 103, the large number of heat source tubes 104, and the outlet chamber 102 successively in the order named. Of this route, the heat source tubes 122 are disposed in the NG heater E3 and the heat source tubes 104 are disposed in the intermediate fluid evaporator E1. The outlet chamber 112 of the LNG evaporator E2 is connected to the second shell 120 side of the NG heater E3 through an NG conduit 123.
In such a vaporizer, sea water as a heat source fluid flows into the outlet chamber 102 after passing through the inlet chamber 121, the heat source tubes 122, the intermediate chamber 103, and the heat source tubes 104. While passing through the heat source tubes 104, the sea water is subjected to heat exchange with the intermediate fluid 105 of liquid phase in the intermediate fluid evaporator E1, thereby evaporating the liquid intermediate fluid 105. On the other hand, LNG to be vaporized is introduced to the heat transfer tubes 113 through the inlet chamber 111. The evaporated intermediate fluid 105 condenses with heat exchange between the LNG in the heat transfer tubes 113 and the intermediate fluid 105 of gaseous phase in the intermediate fluid evaporator E1. By receiving heat generated upon condensation of the gaseous intermediate fluid 105, the LNG evaporates and becomes NG in the heat transfer tubes 113. The produced NG is introduced to the NG heater E3 from the outlet chamber 112 through the NG conduit 123, and is further heated with heat exchange between the NG and the sea water flowing through the heat source tubes 122 in the NG heater E3. Thereafter, the NG is supplied to consumers.
With the intermediate fluid type LNG vaporizer having the above-described construction, LNG can be continuously vaporized through repeated evaporation and condensation of the intermediate fluid 105.
In most of intermediate fluid type vaporizers that have been conventionally used, the heat source fluid is sea water. In some of stations employing intermediate fluid type vaporizers, however, another heat source fluid such as warm water or an aqueous solution of glycol has become used in a place where sea water cannot be used from the standpoint of environmental protection, or in the case where sea water is not used to combine the cold heat recovery system.
In a conventional intermediate fluid type vaporizer using sea water as a heat source, a temperature difference obtainable with sea water as a heat source for vaporization is in the range of 5-7xc2x0 C. Meanwhile, in an intermediate fluid type vaporizer using another heat source fluid such as warm water or an aqueous solution of glycol instead of sea water, a relatively large temperature difference of about 20xc2x0 C. can be utilized for vaporization.
In the latter vaporizer, therefore, a flow rate of the heat source can be reduced. However, the heat transfer efficiency is deteriorated because a flow speed of the heat source flowing through the heat source tubes 104 in the intermediate fluid evaporator E1 and the heat source tubes 122 in the NG heater E3 cannot be set to a sufficiently high value. Thus, it has been found that, in order to compensate for such a deterioration of the heat transfer efficiency, the overall size of an intermediate fluid type vaporizer must be enlarged, and the cost of a heat exchanger is increased.
One conceivable method for increasing a flow speed of the heat source in the heat source tubes is to reduce the number of the heat source tubes 104 in the intermediate fluid evaporator E1 and the number of the heat source tubes 122 in the NG heater E3. However, reducing the number of the heat source tubes decreases a heat transfer area and hence gives rise to another necessity of increasing the lengths of the heat source tubes 104 in the intermediate fluid evaporator E1 and the heat source tubes 122 in the NG heater E3. This means that, since the intermediate fluid evaporator E1 and the NG heater E3 are connected to each other in series as shown in FIG. 6, the above method requires a longer installation area in the longitudinal direction, impedes free layout in design due to restrictions imposed on an equipment layout plan at a factory site, and eventually needs a larger land for installation.
In consideration of the state of the art set forth above, it is an object of the present invention to provide an intermediate fluid type vaporizer which employs a heat source fluid capable of providing a relatively large temperature difference utilizable for vaporization, and which can make an overall size of the vaporizer more compact.
To achieve the above object, an intermediate fluid type vaporizer according to the present invention comprises an intermediate fluid evaporator constructed by providing heat source tubes in a shell, which contains an intermediate fluid therein, to evaporate the intermediate fluid of liquid phase with heat exchange between the heat source fluid and the liquid intermediate fluid, and a liquefied gas evaporator constructed by providing heat transfer tubes in the shell to evaporate liquefied gas with heat exchange between the liquefied gas and the evaporated intermediate fluid. The heat source tubes are formed by a plurality of straight tubes, i.e., straight tubes arranged so as to constitute two or more passes.
With the above features, by employing a heat source fluid that is capable of providing a relatively large temperature difference utilizable for vaporization, the required flow rate of the heat source fluid can be reduced. Also, by arranging the heat source tubes of the intermediate fluid evaporator so as to constitute two or more passes, a flow speed of the heat source fluid in each heat source tube can be increased, whereby the heat transfer efficiency is enhanced and a sufficient heat transfer area can be ensured. Therefore, a more efficient and compact heat exchanger can be realized. Further, since the two or more passes of the heat source tubes are constituted by the combination of straight tubes and a return chamber rather than using U-tubes, tube bundles can be arranged in a smaller area, thus resulting in a smaller diameter of the shell and a more compact structure of the vaporizer.
Preferably, the heat source tubes are formed by bundles of straight tubes arranged between tube plates provided at opposite ends of the shell such that the tube bundles are extended to go and return between the tube plates while constituting an even number of passes not less than two. With this feature, inlet and outlet chambers for the heat source fluid can be arranged at one end of the shell, and a return chamber can be arranged at the other end of the shell. As a result, the inlet and outlet chambers for the heat source fluid can be arranged closer to each other.
Preferably, the intermediate fluid type vaporizer further comprises a gas heater for heating gas discharged from the liquefied gas evaporator with heat exchange effected between the discharged gas and the heat source fluid supplied to the intermediate fluid evaporator. In this case, the gas heater can be installed independently of the intermediate fluid evaporator and the liquefied gas evaporator.
More specifically, by arranging the heat source tubes of the intermediate fluid evaporator so as to constitute two or more passes, the heat source tubes of the intermediate fluid evaporator are no longer necessarily arranged in series with respect to the heat source tubes of the gas heater. For this reason, the gas heater can be installed as a separate unit independent of the intermediate fluid evaporator and the liquefied gas evaporator. Therefore, the diameter and length of a shell of the gas heater can be set as appropriate without undergoing limitations imposed by the diameter and length of the shell that is in common to both the intermediate fluid evaporator and the liquefied gas evaporator. Consequently, equipment layout of the vaporizer can be more freely designed.
The gas heater is preferably mounted on the shell. This arrangement enables an overall installation area of the vaporizer to be cut down.
Thus, since the intermediate fluid type vaporizer of the present invention employs the heat source fluid capable of providing a relatively large temperature difference utilizable for vaporization and is constructed with a more efficient and compact structure, it can be suitably used for efficiently vaporizing liquefied natural gas into natural gas and supplying the natural gas to consumers.